ScreenshotTools. Added comment for IScreenshotSize

> https://forum.babylonjs.com/t/define-typescript-interface-for-options-of-method-to-make-screenshots/4713

src/Misc/interfaces/screenshotSize.ts

@@ -1,3 +1,7 @@
+/**
+ * Interface for screenshot methods with describe argument called `size` as object with options
+ * @link https://doc.babylonjs.com/api/classes/babylon.screenshottools
+ */
 export interface IScreenshotSize {
   /**
    * number in pixels for canvas height